The present invention relates to an image sensor provided in a facsimile transmitter for reading an image on the subject copy.
A conventional image sensor comprises a charge coupled device (CCD) or an array of self-focusing rod lenses. Consequently, it is difficult to largely reduce the space taken up by the sensor and hence the size of the facsimile equipment. Accordingly, in order to provide a compact facsimile equipment, there has been proposed a contact-type image sensor where the distance between a sheet of subject copy and the sensor is reduced.
Japanese Patent Application Laid-Open 2-202056 discloses one of the contact-type image sensors where a light-emitting device and a photodetector are mounted on the same surface of the substrate, thereby reducing the size of the image sensor. Such an image sensor is shown in FIG. 9. The image sensor has a base plate 1 made of glass, ceramics or metal, on which a light-emitting unit 2 and an array of photodetector units 3 are mounted. The light-emitting unit 2 comprises an insulting substrate 2a, lower electrode 2b, light-emitting film 2c, and an upper electrode 2d, laminated subsequently one after the other.
Each photodetector unit 3 has an amorphous-silicon photoelectric transducer film 3c which is interposed between a common electrode film 3b disposed on an insulting substrate 3a and a discrete electrode 3d. The light-emitting unit 2 and the photodetector units 3 are covered with a protecting film 4 made of silicon or resin. A sheet 5 of subject copy, the image of which is to be transmitted, travels above the film 4.
In operation, a beam of light is emitted from the light-emitting film 2c to the sheet 5 passing through the protecting film 4. The beam is reflected on the sheet 5 and applied to the photodetector units 3. The light beam is converted into electric energy, namely to electric current, by the photoelectric transducer film 3c and transmitted to an external member through discrete electrode 3d. Thus, the image is read so as to be transmitted to a facsimile receiver.
Referring to FIG. 10, in another type of contact-type image sensor, a LED bar array 2e as a light-emitting means is provided above the image sensor body. The image sensor has a glass base substrate 1a, on the underside of which is mounted the common electrode film 3b and the photoelectric transducer film 3c. The discrete electrodes 3d each comprising a transparent conductive film 3e and a metal film 3f are further formed on the substrate 1a. A hole 2f is formed in each laminated photodetector unit 3, thereby allowing light beams from the LED bar array 2e, which is disposed over the glass substrate 1a, to reach the sheet 5. Resin 4a is filled in a space between the photodetector unit 3 and a protecting glass plate 4c. In order to maintain the distance between the transparent conductive film 3e and the sheet 5 constant, a plurality of resin particles 4b are embedded in the resin 4a.
An untransparent material is applied on the inner periphery of the hole 2f to form a coating 3g so that the direct beam from the LED bar array is prevented from being sensed by the photoelectric transducer film 3c.
As the sheet 5 is carried by a feed roller 5a in a direction D, the light beams from the LED bar array 2e are introduced through the holes 2f and reflected on the sheet 5. The quantity of the reflected light beams, that is difference of lightness on the surface of the sheet 5 is detected by each photodetector unit 3, and applied to an external member through the corresponding discrete electrode 3d.
In order to accurately read the image on the sheet 5, it is necessary to maintain the distance between the sheet 5 and the photodetector unit 3 constant. However, in the conventional image sensor, the sheet 5 is supplied and passed through the image sensor in parallel to the substrate 1 while trying to maintain the contact with the surface of the protecting film 4. As a result, the sheet 5 is apt to flutter, thereby varying the distance between the sheet and the photodetector unit 3. Hence the difference in lightness is affected so that only a poor reading accuracy is achieved.
Such fluttering can be restrained to a certain extent when the feed roller 5a urges the sheet 5 against the glass plate 4c as shown in the conventional image senor of FIG. 10. However, the force cannot be evenly exerted on the sheet along the whole axial length of the roller, thereby rendering it difficult to maintain the same distance between the sheet 5 and the photodetector unit 3. Moreover, a device for precisely adjusting the force applied by the feed roller 5a must be provided so that the construction of the facsimile transmitter becomes complicated.
In addition, since the image sensor is laterally disposed, a large space is occupied, so that the size of the facsimile equipment cannot be reduced. More particularly, in the image sensor shown in FIG. 9, a cumbersome operation of mounting the light-emitter unit 2 and the photodetector units 3 on the substrate is required. In the sensor shown in FIG. 10, the need of the coating 3g, for example, makes the construction of the photodetector unit complicated, thereby increasing the manufacturing cost of the image sensor.